This invention relates to a method and apparatus for clarifying aqueous liquids containing suspended contaminants, and more particularly pertains to a method and apparatus for removing suspended contaminants from a liquid by utilizing a specific gravity modification technique for increasing the separation and efficiency of the suspended contaminants from the liquid.
In the production of petroleum, varying amounts of water are usually co-produced which may be separated from the crude petroleum by a variety of physical and/or chemical separation techniques. The separated produced water normally contains varying amounts of oil dispersed throughout in the form of extremely small microscopic oil droplets, emulsions, oil-coated solids and the like and other contaminants that are suspended in the water which do not respond favorably to conventional separation techniques, especially gravity separation. The presence of such suspended oil-type contaminants creates a pollution problem in the disposal of the waters, particularly in remote production areas, such as offshore production areas. It has now become incumbent to further treat these waters to remove as much of the suspended oil-type contaminants as is practically possible to meet stringent Federal and State regulatory environmental standards. Similar problems exist in the disposal of petroleum effluents, chemical plant effluents and like industrial effluents as well as many types of discharges from food processing industries that contain suspended animal fats, vegetable oils, organic matter and the like as contaminants or pollutants.
There are several methods known for removing residual oil contaminants from produced waters, refinery effluents, and similarly contaminated industrial effluents. For example, several filtration techniques are known which include the employment of a wide variety of filtration media such as sand, pumice, perlite, screen-type filters, and the like for removing the suspended oil particle contaminants. Several coalescing techniques are also known in which a porous media is used to enhance oil droplet growth and gravity separation. However, these filtration and coalescing methods suffer from several disadvantages including the requirements of back-washing and/or frequent removal and replacement or cleaning of coalescing or filtration media which leads to high maintenance costs and further waste disposal problems.
Several gas flotation techniques and apparatus are also known which generally include intimately mixing a suspended oil particle contaminated liquid with gas bubbles which aids the gravitational separation of the suspended oil contaminants from the liquid. Types of gas flotation systems may be generally classified as full-stream pressurization, split-stream partial pressurization, partial recycle pressurization and multicell impeller-eductor systems.
In full-stream pressurization systems a pressurized contaminated water stream is exposed to pressurized gas under relatively quiescent conditions, dissolving a small amount of the gas into the water stream. This pressurized gas-contaminated water stream is then injected into a flotation tank where the gas is flashed out of solution in the form of bubbles which float the suspended oil contaminants to the water surface in the tank. The flotation tank usually includes a device for removing the floating oil contaminants from the surface of water. These systems are usually once-through systems, i.e., the contaminated waters are treated only once.
Split-stream partial pressurization and partial recycle systems are substantially similar to the full-stream pressurization systems except that the latter systems normally include mixing only a portion of the contaminated water with gas or mixing a portion of the effluent from the flotation tank with gas. In such systems, a main stream of oil-contaminated water is injected into a flotation tank in close proximity with the partial pressurized water-gas stream for intimate admixture with the gas bubbles formed.
Impeller-educator systems usually include educting a gas stream into multiple flotation cells in close proximity to an impeller which imparts shear to the water to create a dispersion of gas bubbles. An oil-contaminated waste water stream in such systems is injected into the flotation cells in close proximity to the impeller and educted gas bubbles.
For a more detailed description of the above-mentioned gas flotation systems, see M. M. Ellis et al., "Clarifying Oilfield and Refinery Waste Waters by Gas Flotation," Journal of Petroleum Technology, (Apr., 1973), 426-430.
However, gas flotation systems presently available leave much to be desired in clarifying many types of suspended oil-contaminated waters and similar contaminated liquid effluents. Known systems usually require the use of multiple flotation cells to successfully clarify these waters sufficiently to meet regulatory environmental standards. Multiple flotation cells or tanks require excessive space and excessive load support that make them uneconomical and impractical in many areas where their use is required such as remote petroleum production areas and offshore production platforms. In addition, multiple flotation cells require the utilization of multiple power units for their operation which increases maintenance and power costs.
We have invented a new and improved method and apparatus for removing suspended contaminants from liquids such as petroleum production waters, refinery effluents, chemical plant effluents, food processing plant waste waters, and the like, without creating additional disposal problems and the use of multiple flotation cells or tanks. The invention is based upon a novel application of a gas flotation technique and includes a novelly designed compact single flotation cell apparatus which enables the contaminated liquid to be treated repeatedly, thus providing multiple performance. The inventive apparatus requires only a relatively small amount of space, power and maintenance and is particularly useful in remote production areas such as offshore productions areas.